1. Technical Field
This invention relates generally to laser systems and, more particularly, to a laser resonator for producing a radially polarized laser beam.
2. Discussion
The polarization of a laser beam is a characteristic in which the electric field thereof is controlled usually with respect to the propagation direction of the beam. Different types of polarization exist which are commonly selected so that the electric field is oriented in a predictable fashion, depending on the particular application. For instance, conventional laser systems have been developed which commonly generate a linearly polarized beam. According to such conventional approaches, the linear polarization is generally accomplished by employing a flat planar Brewster window oriented at the Brewster angle (i.e., polarizing angle) within the lasing region. The planar Brewster window is typically made of a transparent dielectric material such as glass or dichroic material which has a known index of refraction. Accordingly, light with a polarization parallel to the incidence plane of the Brewster window is transmitted therethrough, while light with a polarization normal thereto is generally reflected therefrom.
In the past, other types of polarization have been produced which include circular polarization and elliptical polarization, which are normally produced by combining a linear polarizer with a wave plate such as a quarter wave plate or Fresnel rhomb. However, in more recent years, developments in the areas of holography, interferometry, spectroscopy, photochemistry and accelerator technology now and in the future may require laser beams in a special polarization state known as radial polarization. In particular, a radially polarized laser beam can advantageously be focused by axicon-type optics so as to generate a very strong longitudinal electromagnetic field in the focal region thereof. In effect, radially polarized laser beams could therefore be used to provide an accelerating mechanism for present and future-generation high energy accelerators.
Currently, existing laser systems have been able to generate radial polarization beams to a limited extent by using very complex optical schemes. Such optical schemes generally involve converting a linearly polarized beam into a radially polarized beam through a series of beam rotations and combinations with external conversion systems. However, such external conversion systems are rather complicated since the conversion generally requires special optical elements such as a spiral waveplate which is generally very difficult to fabricate in the optical region. In addition, prior approaches generally require a substantially uniform beam profile which in turn results in rather stringent requirements.
It is therefore desirable to provide for an enhanced approach for obtaining a laser output beam which has a radial polarization. More particularly, it is desirable to provide a laser resonator that produces a direct radial polarization laser output beam without the need for any complex external conversion optics. In addition, it is desirable to provide for such a direct radial polarization laser output beam which may be easily achievable with existing laser systems by equipping such systems with a conical Brewster window as described hereinafter.